Such containers are also known as filter cartridges, and they are generally used in gravity-operated filter devices. Such filter devices are primarily used in the household, especially for filtration of water.
The filter cartridges are loaded with filter materials, especially in granular form.
In the filter device, the filter cartridges are inserted into a water funnel, into which the water being filtered is poured. The water funnel is placed in a jug, which catches the filtered water.
The filter cartridges are only temporarily used, namely, when filtered water is required. At the very start of the filtration process the filter cartridges must assure a flawless passage of the water, on the one hand letting the air contained in the filter cartridge escape unhindered and on the other hand preventing the filter material from getting into either the water prior to filtering or the filtered water.
A filter cartridge is known from EP 0 823 276 B1, which has a cup partially filled with filter material and a cover placed on the cup. The cover is in the form of a hood and it has air venting slots in its sides and water inlet openings in the form of a crown. To prevent small granular particles from getting through the openings in the cover to the outside and possibly blocking the openings in the cover, a fabric insert with shape-holding ribs is provided, which is fastened in the region of the edge of the cover and protrudes into the interior of the cup. To break the surface tension, the fabric insert makes contact with the filter material. At this contact site, the liquid begins to flow and forces out any air cushion underneath the cover, thus preventing any back pressure opposing the pressure of the liquid column. This is supposed to let the liquid being filtered flow without problems through the filter material and hold back the small particles of the filter material in excellent manner.
In WO 01/32560, however, it is stated that flow problems occur if contact between fabric insert and filter material is not assured.
On the one hand, drying out of the filter material can lower the level of the filter material, so that the desired contact is no longer assured. On the other hand, sometimes too much filter material can be filled into the cup, so that too large a pressure is exerted on the inwardly projecting fabric insert, and this is further intensified by a vigorous expansion of the filter material when exposed to hard water, which can lead to cracking of the filter cartridge.
To obviate this problem, it was proposed in WO 01/32560 to provide for a flexible, screenlike, textile sheet structure instead of the rigid fabric insert, projecting upward into the cover when water flows in. As the water level drops, the sheet structure is pulled downward.
The solutions in both documents have the drawback that a screenlike fabric must be provided, spanning the entire opening of the cover, in addition to the windows in the cover. Neither of the two solutions has satisfactorily eliminated the sail effect and thus the hindering of the flow.